It's a warm and humid start to the week but we are going to see that break down with some thundery showers by midweek. Thereafter temperatures will begin to fall somewhere nearer to normal.
High pressure is close by at the moment but it'll move away to the east allowing weather systems in, but also help to drag warm air in from the south. These things combined will give rise to some potentially hefty downpours from as early as Tuesday evening.
It'll be dry this evening though, and overnight too, but cloud will increase during the small hours of Tuesday bringing some mist and hill fog. It'll be uncomfortably warm with temperatures not much lower than 15 or 16°C.
Cloud will clear fairly quickly on Tuesday morning allowing plenty of strong and warm sunshine through. By the afternoon temperatures will have risen into the low 30s for some! Cornwall will more likely be around 25°C. For the rest of the week there is a continued chance of showers, especially towards Friday and the weekend, by which point temperatures will be much nearer normal.